The present application claims priority to Japanese Application No. P11-309153 filed Oct. 29, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a recording medium that is read by a laser beam utilizing a magneto-optical effect. More particularly, the present invention relates to a magneto-optical recording medium comprising at least a reproduction layer and a recording layer, on which a readout process based on magnetically induced super resolution is carried out utilizing a blue-violet laser beam, that is, a laser beam having a wavelength of 350 nm to 450 nm.
2. Description of the Related Art
There is a method for rewritably recording a data signal with high density using a magneto-optical recording means. This method comprises utilizing the thermal energy of a laser beam to initially cause a partial portion of a magnetic layer of a magneto-optical recording medium to be heated beyond the Curie temperature or the compensation temperature. The method also includes causing the coercive force in the heated portion to be decreased or extinguished, and then causing the direction of magnetization of the heated portion to be inverted into the direction of a recording magnetic field that is externally added to form an information magnetic domain, so as to execute the recording of the data signal.
The above-cited magneto-optical recording medium comprises a transparent substrate, such as a polycarbonate resin, having a main surface. The recording medium further comprises a plurality of layers sequentially laminated on the main surface of the substrate. The sequentially laminated layers comprise: a dielectric film that may be made of silicon nitride or aluminum nitride; a magnetic recording layer that may be made of an amorphous film of rare-earth transition-metal alloy and that is provided with an easy axis of magnetization in the vertical direction with regard to a film surface and that has a surpassing magneto-optical-effect characteristic. The plurality of layers further includes another dielectric film that may be made of silicon nitride or aluminum nitride; a reflection layer composed of aluminum, gold, or silver; and a protection layer, which may be made of an ultraviolet-ray-cured type resin.
By causing the above-cited magneto-optical recording medium to be exposed to a laser beam irradiated from the side of the transparent substrate, the above-cited information magnetic domain may be formed, such that the recording of the data signal is executed against the magnetic layer.
To reproduce the recorded data signal, the magneto-optical recording medium is exposed to a laser beam irradiated from the side of the above-cited transparent substrate to detect the actual rotating angle of the magnetized or polarized surface via a magneto-optical effect, such as the Kerr effect, which is generated in the information magnetic domain in the magnetic layer to execute reproduction of the recorded data signal.
The linear recording density of optical disks (such as the magneto-optical recording medium, a disc for digital audio recording, a disc for digital video recording or the like) is also determined by the signal-to-noise (S/N) ratio in the course of performing reproduction in most cases. And yet, the amount of the recorded data signal read under reproduction mode is dependent on the cyclic period of bit array of the recorded data signal, the wavelength (xcex) of the laser beam, and the numerical aperture (N.A.) of the objective lens.
More specifically, the bit cyclic period (xe2x80x9cfxe2x80x9d) is determined by the laser wavelength (xcex) of the reproduction optical system and also by an optical detection limit of the optical system. The optical detection limit may be an optical refraction limit determined by the numerical aperture (N.A.) of the objective lens. More precisely, the bit cyclic period (xe2x80x9cfxe2x80x9d) at the optical detection limit is defined by equation expressed by way of xe2x80x9cf=xcex/(2 N.A.)xe2x80x9d.
Accordingly, in order to realize higher density recording of a data signal in the magneto-optical recording medium, the laser wavelength (xcex) of the reproduction optical system may be shortened or the numerical aperture (N.A.) of the objective lens may be increased. However, within the current technology, improvement of the laser wavelength (xcex) of the laser beam and the numerical aperture (N.A.) of the objective lens are limited.
In recent years, modem technology has developed a semiconductor laser having a wavelength of around 400 nm. For example, a GaN semiconductor laser is capable of producing a blue-violet laser beam having a laser wavelength of 350 nm to 450 nm. On the other hand, a numerical aperture (N.A.) of an objective lens that is about 0.7 may be obtained by a resin mold.
In view of the above-described circumstances, there have been a number of studies, developments, and suggestions with regard to structures of a magneto-optical recording medium and methods for recording and reproducing such a medium.
As for the recording method, a so-called xe2x80x9cmark length recording methodxe2x80x9d has been suggested. According to the xe2x80x9cmark length recording method,xe2x80x9d an information mark is recorded not by a method in which information is recorded according to the presence of a mark (i.e., the so-called xe2x80x9cmark position recording methodxe2x80x9d), but by a method in which a linear density of the recording medium is achieved by varying the length of the mark to make an edge portion thereof to record the information.
Another recording method, xe2x80x9ca laser pulse irradiation magnetic field modulating recording method,xe2x80x9d has also been suggested. According to this recording method, a recording laser beam is irradiated so that a waveform thereof is made to be a pulse form, not a continuous waveform, in accordance with a phase of an external magnetic field. This method results in prevention of unnecessary expansion in a track direction of the recording medium, reduction of cross-write and cross-erase in adjacent tracks, and improvement in track density.
On the other hand, there have been suggested a number of xe2x80x9cmagnetically induced super resolution readout methods,xe2x80x9d as a reproducing method.
In order to realize the above xe2x80x9cmagnetically induced super-resolution readout methodxe2x80x9d, a magneto-optical recording medium comprising at least a reproduction layer and a recording layer is introduced. This method comprises a process for causing the data signal recorded on the recording layer, which may have high coercive force, to be transferred onto the reproduction layer. In this method, a polarized surface is subject to rotation by a magneto-optical effect (e.g., Kerr effect) of the reproduction layer generated by a laser beam irradiated onto the reproduction layer. By detecting the rotation of the polarized surface, readout (reproduction) of the recorded data is executed. In this case, by using the reproducting laser beam to form a thermal distribution within a spot in the reproduction layer, a part of the recorded data signal is emerged in the spot of the reproduction layer in order that the optical reader can restrictively read a single information magnetic domain within the spot, thus making it possible to reproduce the information magnetic domain based on a cyclic period being less than the bit cyclic period at the above-described optical detection limit.
The magnetically induced super-resolution readout method described above is disclosed in Japanese Patent Application Publication Laid-Open No. HEISEI-1-143041/1989 and also in Japanese Patent Application Publication Laid-Open HEISEI-1-143042/1989, both being a basic application of the U.S. Pat. No. 5,018,119. The magneto-optical recording medium used for implementing the magnetically induced super-resolution readout method essentially comprises a reproduction (readout) layer, an intermediate layer, and a recording layer, which are magnetically coupled with each other at room temperature. When performing the magnetically induced super-resolution readout method, the recorded information magnetic domain of the reproduction layer heated within the irradiated laser beam spot expands or contracts or inverts at a portion bearing higher temperature, resulting in a decrease in interference between data marks during the reproduction process. This decrease in interference makes it possible to perform the reproduction process based on a cyclic period being less than the limit of optical refraction of the optical system, enabling linear recording density and track-recording density to be increased as is proposed by the above-disclosed inventions.
Further, as a part of the above-described magnetically induced super resolution readout method, there is another art called xe2x80x9cCenter Aperture Detection type Magnetic Super Resolutionxe2x80x9d (xe2x80x9cCAD-MSRxe2x80x9d hereinafter) that is used primarily to read the recorded bit (i.e., the magnetized recording domain) at the center portion of the reproducing laser-beam spot based on the method disclosed in the Japanese Patent No. 2839783 (Japanese Patent Application Publication Laid-Open HEISEI-5-081717/1995), a basic application of U.S. Pat. No. 5,707,727.
FIG. 2 depicts a schematic cross-sectional view of an exemplary magneto-optical recording medium of the present invention that may be used to implement the above-identified CAD-MSR readout method. The magneto-optical recording medium shown in FIG. 2 comprises the following: a transparent substrate 10 (which may be made-of polycarbonate resin) on which a first dielectric film 5, a reproduction (readout) layer 1, an auxiliary reproduction layer 2, a non-magnetic intermediate layer 4, a recording layer 3, a second dielectric film 6, and a thermal-control layer 7 are laminated in the upward direction. Further, a protection layer 8 may be superficially formed on the top surface of the thermal-control layer 7.
FIG. 3 depicts a schematic cross-sectional view of a portion of the magneto-optical recording medium in FIG. 2, which comprises the reproduction (readout) layer 1, the auxiliary reproduction layer 2, the non-magnetic intermediate layer (cut-off layer) 4, and the recording layer 3. Of these, both the reproduction layer 1 and auxiliary reproduction layer 2 are respectively composed of a magnetic layer having in-plane magnetic anisotropy. The recording layer 3 comprises a magnetic layer having perpendicular magnetic anisotropy, whereas the non-magnetic intermediate layer 4 comprises a non-magnetic-material layer.
As shown in FIG. 3, a recorded mark M is recorded in response to recording a data signal, such that a corresponding information magnetic domain, is formed (i.e., recorded) on the recording layer 3.
Note that outline arrows shown in FIG. 3 schematically show the direction of magnetization.
Reading (in other words, reproduction) a recorded data signal from the magneto-optical recording medium based on the above-referred CAD-MSR method is carried out by irradiating the magneto-optical recording medium with a reproducing laser beam L through an optical lens 11, i.e., an objective lens. Due to the irradiation by the reproducing laser beam L, the magnetized effect disappears from the heated portion I of the auxiliary reproduction layer 2 to cause a reproducing window W, i.e., an aperture, to be formed in the auxiliary reproduction layer 2. The reproducing window W defines a demagnetized portion in the layer 2. The reproduction layer 1 may then be magnetically coupled with the recording layer 3 via the reproducing window W to allow the recorded mark M to be transferred onto the reproduction layer 1. Next, the transferred recorded mark M may cause the reproducing laser beam to generate the Kerr rotation. By detecting the actual angle of the Kerr rotation performed by the returning laser beam, the recorded data signal may be read.
As was described earlier, while performing reproduction of a variety of data in accordance with the magnetically induced super-resolution readout method by lowering the limit of the refraction of a blue-violet laser beam having a wavelength of 350 nm to 450 nm, it is possible to secure high density that is more than double the conventional magnetically induced super-resolution readout method, which uses a conventional red laser beam having a wavelength of approximately 600 nm to 800 nm.
Nevertheless, when utilizing such a blue-violet laser beam, the signal amount (carrier level) is lowered, which raises problems such as a decrease in carrier-to-noise (C/N) ratio and an increase in the amount of jitter.
A rare-earth transition-metal magnetic film utilized as a conventional magneto-optical recording medium (for example, a thin amorphous alloy film of TbFeCo or GdFeCo) is known to have an undesirable characteristic feature where the Kerr rotational angle or the Faraday rotational angle (both exhibiting a magneto-optical effect that is in proportion with a signal amount (carrier level) of the magneto-optical recording medium) is decreased in a region of a blue-violet laser beam having a wavelength of 350 nm to 450 nm.
In addition, a conventional Si photo-detector for detecting a reproducing beam is known to have a characteristic feature where a signal amount (carrier level) at the time of converting optical information obtained from magnetized information recorded in the recoding medium into an electrical signal is decreased (i.e., photo-detector sensitivity decreases) due to deterioration of quantum efficiency in the wavelength region of 350 nm to 450 nm.
Accordingly, as described above, when performing the magneto-optical recording/reproducing method utilizing a blue-violet laser beam having a wavelength of 350 nm to 450 nm an improvement in the limit of optical detection may be realized due to the shorter wavelength of the blue-violet laser beam. But performing this method using the blue-violet laser beam also causes deterioration in the magneto-optical effect due to the shorter wavelength of the blue-violet laser beam and deterioration in a signal amount (carrier level) due to deterioration in photo-detector sensitivity, which results in deterioration in carrier-to-noise (C/N) ratio and increase of jitter.
The C/N ratio and the jitter at the time the mark signals are randomly recorded depend on various conditions, such as compositions, film thickness, film forming conditions or the like of respective magnetic layers of the magneto-optical recording medium. However, even if these conditions are optimized, the C/N ratio and the jitter are deteriorated in a case where the magnetically induced super resolution readout is carried out by utilizing a blue-violet laser beam, in comparison with a case of utilizing a conventional red laser beam having a wavelength of 600 nm to 800 nm.
FIG. 4 shows measurement results of carrier level, noise level and C/N in a case of utilizing a red laser beam having a wavelength of 635 nm. FIG. 5 shows the same in a case of utilizing a blue-violet laser beam having a wavelength of 406 nm.
The measurements for both the red laser beam and the blue-violet laser beam were carried out under the following conditions: a numerical aperture (N.A.) of an objective lens of measurement apparatus is 0.6, a track pitch of the magneto-optical recording medium is 0.55 xcexcm, and recorded mark lengths are 0.4 xcexcm and 1.6 xcexcm.
As apparent from FIGS. 4 and 5, in a case of reading out a recorded mark of the same length, the carrier level, noise level and C/N ratio are all deteriorated when utilizing the blue-violet laser beam.
A method and an apparatus consistent with the present invention provides a magneto-optical recording medium capable of effectively overcoming the above-described problems associated with using a blue-violet laser beam for readout of the recording medium, among other problems.
The present invention provides a magneto-optical recording medium having at least a reproduction layer and a recording layer on a substrate thereof, and on which data-reproduction based on a magnetically induced super-resolution readout method utilizing a blue-violet laser beam is carried out. The substrate has a base surface on which the reproduction layer and the recording layer, that is, recording/reproducing magnetic layers, may be formed, or on which a dielectric film is formed (this dielectric film is referred to as the xe2x80x9cfirst dielectric filmxe2x80x9d hereinafter) such that the dielectric film has a surface roughness (Ra) of 0.3 nm or less. It is noted that xe2x80x9cRaxe2x80x9d means an average center roughness as defined in JIS B0601, and is merely referred to as xe2x80x9csurface roughnessxe2x80x9d in this specification.
Recording on the magneto-optical recording medium, so as to form an information mark (magnetic domain) according to the present invention may be carried out in accordance with a light intensity modulation or a magnetic field modulation.
Reproduction (readout) of a recorded data signal on the magneto-optical recording medium is carried out in accordance with the magnetically induced super resolution readout process, preferably utilizing a blue-violet laser beam. In the present invention, the above-described surface roughness is selected for a preferred structure, which reduces noise, and thus, improves the C/N ratio and the jitter associated with recording or reproducing of the data signal on the magneto-optical recording medium.